On the Surface Swarming of Euphausiid Crustaceans

Abstract

A general aspect of the daytime surface swarming of Euphausia pacifica in Japanese nearshore waters is described in connection with the water temperature. Swarming usually starts with a local minimum temperature around 7°C and terminates with a temperature just below 16°C. The swarming season is essentially in spring, from February through May, with little difference among regions. The main swarming areas are on the Pacific coast around Kinkazan, and on the coast of the Sea of Japan around Sadogashima, in Wakasawan and its vicinity, around Oki, and on the east side of the Tsushima Gunto. It is shown that the swarming is closely related to cold water masses, and that the approach of offshore cold water masses to the nearshore areas and the mixing process in the coastal areas may provide favorable conditions for swarming . Swarming of E. pacifica is a phenomenon that occurs at the margins of the cold water bodies, and is related to the seasonal change in the geographical distribution of those euphausiids. A uniformly low water temperature profile must be the necessary condition for swarming, but other possible factors stimulating euphausiids to swarm are enumerated.